How do I format a number with commas?

Question

Try N0 for no decimal part:

string formatted = a.ToString("N0"); // 10,000,000

You can also do String.Format:

int x = 100000;
string y = string.Empty;
y = string.Format("{0:#,##0.##}", x); 
//Will output: 100,000

If you have decimal, the same code will output 2 decimal places:

double x = 100000.2333;
string y = string.Empty;
y = string.Format("{0:#,##0.##}", x); 
//Will output: 100,000.23

To make comma instead of decimal use this:

double x = 100000.2333;
string y = string.Empty;
y = string.Format(System.Globalization.CultureInfo.GetCultureInfo("de-DE"), "{0:#,##0.##}", x);

a.ToString("N0")

See also: Standard Numeric Formatting Strings from MSDN

A simpler String.Format option:

int a = 10000000;
String.Format("{0:n0}", a); //10,000,000